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Feb. 17, 2011 -- Drugs prescribed to prevent fractures in osteoporosis may do double duty, cutting a woman’s risk of colon cancer by more than half for those who take them for at least a year, a new study shows.

The study is the latest in a growing body of research suggesting that bisphosphonates, drugs that lower fracture risk by slowing bone breakdown and increasing bone mass, may also fight cancer.

A pair of studies published last year in the Journal of Clinical Oncology, one by the same group responsible for the colon cancer finding, found that bisphosphonate use was associated with about a 30% reduced risk of getting breast cancer.

“That was a bit of a surprise because these drugs were not thought to have effects on cancer cells per se,” says Mone Zaidi, MD, PhD, professor and director of The Mount Sinai Bone Program at Mount Sinai School of Medicine in New York. Zaidi is studying the effects of bisphosphonates on cancer cells in the lab, but he was not involved in the current research.

“These are new actions of old drugs, and I think it’s actually quite a dramatic action of a widely used drug in osteoporosis that could have potential implications in cancer,” Zaidi says. “This study has profound implications.”

But other experts, as well as the study’s researchers, urged caution, noting that the study was powered only to demonstrate an association, not an action.

The researchers could not rule out, for example, that women who were motivated to take drugs to prevent fractures weren’t similarly motivated to take better care of their overall health, which might also have reduced their risk of cancer, says Rowan T. Chlebowski, MD, PhD, chief of medical oncology and hematology at the David Geffen School of Medicine at UCLA. “It’s still a question,” he tells WebMD.

Chlebowski is analyzing data from the Women’s Health Initiative study to see if he might also find ties between bisphosphonates and reduced risk of colon cancer. He was not involved in the current research.

Bisphosphonates and Cancer Risk

For the study, which was published online in the Journal of Clinical Oncology, researchers from Israel and the U.S. recruited 933 postmenopausal women with colon cancer and matched them by age, ethnicity, and location to a group of healthy women.

The researchers then looked at pharmacy records to determine which women had used bisphosphonate medications and for how long those drugs were used.

They found that women who had used the drugs for at least a year had a 50% reduced risk of colon cancer compared to those who had taken them for shorter periods.

That association actually got slightly stronger, showing a 59% risk reduction, even after researchers took into account other things known to influence colon cancer risk, including family history of colon cancer, body mass index (BMI), sports activity, eating vegetables, and the use of vitamin D, aspirin, statins, or hormone replacement therapy.

“I would not recommend bisphosphonates for prevention [of cancer] to anybody at this stage,” says study researcher Gad Rennert, MD, PhD, of the Technion-Israel Institute of Technology faculty of medicine and chairman of the department of community medicine and epidemiology at the Carmel Medical Center of Clalit Health Services.

“My study is an association study and needs to be replicated by others and potentially brought to test in a randomized, controlled trial before we can recommend that. This is how our knowledge evolves, step by step. My study is the first to suggest this association and I hope more will follow and we will find ourselves with a new drug with cancer prevention qualities,” Rennert says.

Rennert points out, however, that the association is strengthened by the fact that other groups have noted similar effects on cancer and because there’s some evidence to suggest that bisphosphonates may have a direct effect on the biology of cancer cells.

In petri dishes, bisphosphonates have been shown to having a wide range of anticancer effects. “In the next six to nine months you’re going to see more literature on the epidemiology involved and the basic science involved,” Zaidi says. “This is not happening in isolation. It’s real.”

In the meantime, he adds, patients who need bisphosphonates for their bones should feel good that these drugs may have added benefits.

“That’s a very nice thing for people to hear, particularly those who have a family history of colon cancer.”